This invention relates to the measurement of strain in structures by the use of electrical strain gauges.
It is usual to mount a series of such gauges on a support that is in its turn mounted on the structure the strains in which are to be measured. This in particular facilitates the measurement process because it is possible to so arrange the gauges that various sources of error, in particular zero errors, are cancelled out, but even if a single gauge is employed it may be preferred to mount it on a support in order to attach it more conveniently to the structure or to provide it in an enclosure that will protect it against dirt and damage, or to provide a magnification of the structure strain.
The gauge is therefore not directly in contact with the structure that is to be monitored and problems can arise in ensuring that strains in the structure are correspondingly reflected in the gauge or gauges, or of providing a unit that can be mounted on the structure without requiring a great deal of effort or skill. The problem is made more difficult by the fact that it is usually necessary to ensure that the unit is secured very firmly to the structure, and yet it must be able to allow the strain gauge or gauges to respond freely to strains of the structure.
For example, in U.S. Pat. No. 2,316,075, there is described a unit comprising an arch-form beam, the legs of which engage the surface to be monitored and strain gauges on the upper and lower edges of the arch are respectively compressed and extended if the surface between the legs of the unit is extended, the gauges being connected into a bridge circuit to evaluate the strains occurring in the structure. Such an arrangement gives a direct response to strains in the structure but it is impossible to exert such pressure on the legs as would hold them firmly in place if, for example, the structure was moving. It is therefore more suited to carefully controllable laboratory conditions than to general industrial application.
UK Patent 1,335,520 is an example of a unit which can be firmly secured in placed and which is moreover adapted to be fully enclosed so that it can be used in instances where dirt or moisture create a problem and it can be mounted securely on a moving structure, such as an axle or wheel support of a vehicle. The unit comprises spaced end blocks interconnected by an integral but flexible carrier element on which the gauges themselves are mounted, the end blocks being provided with means to secure them to the structure to make them each completely fixed with respect to the adjacent region of the structure, so that the carrier element will then deform with movements of the structure.
In such an arrangement, preparation of the surface to receive the unit is more complicated since it is now necessary for two spaced regions of the surface to be made co-planar if the unit is to be secured in place without initial deformation. Also, the gauges are able to produce signals that are only indirectly related to the extension and contraction of the structure surface between the end blocks and they may be influenced by extraneous movements such as twisting of the structure. Because the end blocks are positively fixed to the structure, all of the surface will be transmitted to the gauges, but this can be a disadvantage if the installation is susceptible to overload, e.g. through transient dynamic loading, and damage may then be caused to the gauges or their measuring circuit.